AETIOLOGY OF TUBERCULOSIS. 
213 
single cases of the disease have already often taught that one and the same 
person is not at every time an equally favorable subject for the development 
of the parasites, for, as is well known, it occurs not rarely that tuberculous 
herds which had reached no slight extent, shrivel, make scars and heal. That 
is as much as to say that the same body which, at the invasion of the tuberculous 
bacilli gave a favorable breeding-ground for the same,so that they could increase 
and spread, gradually loses these favorable properties, changes itself into a bad 
breeding-ground and so sets a boundary to the further growth of the bacilli. 
There existed, therefore, in the same person at one time a disposition for tu¬ 
berculosis and at another time not. Wherein this distinction is founded, 
whether in a change in the chemical composition of the juices of the tissue, 
or in physical conditions, that must be taught by later investigations. So much 
is certain, that such differences exist and there is certainly nothing against the 
supposition that similar conditions, favorable or unfavorable to the tuberculous 
bacilli, may exist in certain persons not only for a time, but also during the 
entire life. 
What still concerns the much discussed question of hereditary tuberculosis* 
after what has just been said, I can express in a few words. No facts exist 
which justify the supposition that intra-uterine or extra-uterine tuberculous 
bacilli can be present in the organism of a child, without bringing about visible 
changes in a comparatively short time. But until now tuberculosis has been 
very seldom found in the foetus or in the newly born child, and we may, there¬ 
fore, conclude that the infectious material has effect only exceptionally during 
the intra-uterine life. This supposition is confirmed by the fact that of my 
experimental animals, especially guinea pigs, which not seldom were pregnant 
before or after the tuberculous infection, none have borne young which were 
tuberculous at birth. The young coming from mothers tuberculous to a high 
degree were free from tuberculosis and remained healthy for months. In my 
opinion hereditary tuberculosis finds its most natural explanation, if it be sup¬ 
posed that not the infectious germ itself, but certain qualities favoring the 
development of the germs coming into contact with the body at a later period, 
therefore, that which we call disposition, be inherited. 
The aetiology of tuberculosis, as it was here developed on the foundation 
of our knowledge of the tuberculous bacillus in detail, scarcely offers anything 
new. Cohnheim had represented tuberculosis as an infectious disease and de¬ 
scribed its aetiology correspondingly before the discovery of the tuberculous 
bacillus. In Ihis direction, therefore, my investigations have brought no essen¬ 
tial progress to science, and yet it must be considered as a gain that upon the 
very important question of the infectious nature of tuberculosis, which until 
then had been disputed by most, now such proofs are furnished as to admit 
of no reasonable objections. Not less important is it, that the tuberculous 
bacilli give a sure test of what in the future shall be considered as belonging 
to the territory of tuberculosis. The diagnosis of tuberculosis will in doubtful 
cases be made dependent upon the proof of tuberculous bacilli. Practice has, as 
is well known, made use of this aid to a great extent and moreover with com¬ 
plete success, and has thereby furnished a rich material.for proving the correct¬ 
ness of my opinion of the importance of tuberculous bacilli. Already froin 
